`code` was ambiguous next to Python's own use of "code"; `action_code` names
its actual role (a click-action lookup key). click_object_id_value pairs it
consistently with action_code. Renamed on both PushMessage and
NotificationLink via a data-preserving RenameField migration, threaded
through the serializer, admin, bulk-push path, and docs.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
